Monsanto is reviewing an unsolicited takeover proposal from Bayer AG, a bold attempt by the German company to snatch the last independent global seeds producer and become the world's biggest maker of seeds and farm chemicals.

Bayer AG, the St. Louis-based company, with a market value of $42 billion, is consulting financial and legal advisers and didn't disclose the terms of the proposal. The company confirming the offer said the combination would bolster its position as a ‘global innovation-driven life science company.’

Meanwhile, shares of Bayer dropped amid concern that a large purchase would weigh on its credit rating and force the company to sell more stock. The proposal by Werner Baumann, who has been at Bayer's helm for less than a month, follows Monsanto's failed attempt to buy Syngenta and the proposed merger of Dow Chemical Co and DuPont Co Baumann may need to sell some assets, such as his company's remaining stake in Covestro AG or Bayer's animal health unit, to help fund its quest to buy the world's largest seed maker.

Bayer's stock fell 6.3 per cent, the most in almost nine months, to 90.17 Euros as of 9:23 am in Frankfurt trading. Merging Monsanto with the company that invented aspirin would bring together brands such as Roundup, Monsanto's blockbuster herbicide, and Sivanto, a new Bayer insecticide.